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Old 06-01-2019, 04:54 AM
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Another GM Hydroboost Problem

I didn't want to hijack the other thread but it prompted me to post mine. 2000 chevy 3500 van. Brakes worked fine from new until a few months or more ago. The pedal always had a lot of travel from new but seemed fine. Suddenly the pedal started to grab way up high, like if you press it 2" it's almost locking up the brakes. Then sometimes it would go normal. Even if it was grabbing with the pedal high, it sometimes "slips" with a mechanical bind feel to it and goes down like it used to, so your foot better be ready to take up the slack when it does that.
Somewhere on line I read that it is the symptom of a bad hydro boost unit, replaced it. Same thing. Replaced it again along with the master cylinder and still the same thing. I sort of got used to it and it's been going on for some time now.

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Old 06-01-2019, 07:18 AM
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Does it also have abs with that hydro Boost?
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Old 06-01-2019, 07:24 AM
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Does it also have abs with that hydro Boost?

Yes anti lock. Seems to work fine going down my steep driveway when it's wet. I hit the brakes hard there sometimes because I always think of the abs going bad from not being used. (The problem I have doesn't feel at all like when the abs is working.)
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Old 06-01-2019, 07:31 AM
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Just asked because I read another long thread on another forum about bleeding the abs properly.
Might be some info in here for you.

First hand experience with changing to the NBS master cylinder WOW! - The 1947 - Present Chevrolet & GMC Truck Message Board Network
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Old 06-01-2019, 10:46 AM
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Quote:
Originally Posted by RWENUTS View Post
Just asked because I read another long thread on another forum about bleeding the abs properly.
Might be some info in here for you.

First hand experience with changing to the NBS master cylinder WOW! - The 1947 - Present Chevrolet & GMC Truck Message Board Network
Well, I've owned a 1999 K3500 crewcab dually from new. Over the last 20 years I've had to bleed the brakes a number of times. I've never had an issue with the ABS or needed to do anything special. I use a Vacula vacuum brake bleeder, so maybe that helps, but one pass and the brakes are bled. No muss, no fuss. I've owned four GM one ton trucks with hydroboost and never had an issue with that system, either. Yes, the pedal does seem to travel further than you would expect, but you get used to that. The 99 has been that way since new.
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Old 06-02-2019, 05:52 AM
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Re: Bleeding. The problem arose without any bleeding done to the system in recent history. I did replace both rear wheel cylinders long before the problem arose and I bled the entire system after seeing how bad the fluid looked and the rust in the cylinder. It was one of the easiest vehicles to bleed. Besides, from the factory, like mentioned in another reply, the pedal always had a lot of travel. When the problem started, I suddenly had only a tiny bit of travel. But then sometimes while braking it drops to where it normally was. Most of the time it works like new. It certainly feels like a mechanical problem. Hmm, I wonder if the proportioning valve could be getting stuck off center giving the front all the pressure, then popping back into place? Although I never saw the brake light go on. Could be part way, not enough to trigger the light?
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Old 06-02-2019, 06:45 AM
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Looked up proportioning valves, it's all part of the anti-lock system. I have to see which one mine takes. They run from $160 to $260!
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Old 06-02-2019, 06:50 AM
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Looked up proportioning valves, it's all part of the anti-lock system. I have to see which one mine takes. They run from $160 to $260!
I'm guessing the vans are the same, but my 1999 K3500 doesn't have a "proportioning valve" in the conventional sense. The ABS system looks at the wheel sensors and will pulse the ABS valves to mimic the proportioning function. It's all done in software. There is no mechanical valve with a sliding piston or electrical contact for the BRAKE light. The factory service manual spells this out in detail
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